3D lanthanide-coordination frameworks constructed by a ternary mixed-ligand: crystal structure, luminescence and luminescence sensing

Abstract

Using a mixed ligand synthesis strategy involving semi-rigid 4-(4-carboxyphenoxy)-isophthalic acid (cphtH₃), oxalate (ox) as a mixed linker and 1,10-phenanthroline (phen) as an ancillary terminal ligand, a series of highly stable metal coordination frameworks, Ln-MOFs, [Ln(cphtH)phen(ox)_0.5]_n·mH₂O (Ln = Sm 1, Eu 2, Gd 3, Tb 4 and Dy 5), were synthesized. They are isostructural 3D coordination networks of interlinked left- and right-handed helices based on [LnO₇N₂] connections. The Brunauer–Emmett–Teller (BET) surface area was 3.3020 m² g⁻¹ (Langmuir surface area: 3.6940 m² g⁻¹). They are chemically stable in common solvents and are stable in aqueous solutions from pH = 4 to 9. Complexes 1 and 5 are dual-emissive materials exhibiting a broad emission band from the π*–π transition of the organic linker and narrow emission peaks, the ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, and 9/2) transitions of the Sm³⁺ ion for 1 and ⁴F_9/2 → ⁶H_J (J = 15/2 and 13/2) transitions of the Dy³⁺ ion for 5. Complexes 2 and 4 exhibit red and green emission, assigned to the ⁵D₀ → ⁷F_J (J = 1–2) transitions of the Eu³⁺ ion and ⁵D₄ → ⁷F_J (J = 6–3) transitions of the Tb³⁺ ion, respectively. The maximum excitation wavelengths are 356, 358, 360 and 350 nm for 1, 2, 3 and 4, respectively, which are in the near-visible radiation region (350 nm < λ_ex < 420 nm). The Sm-MOF (1) displays a single component white light based on its dual-emission. The primary focus of this study is to examine the fluorescence properties of Eu-MOF (2) as it showed excellent luminescence in solvent as well as aqueous solutions over a wide pH range (pH = 4–9). Based on the luminescence quenching of the Eu-MOF, the detection of quercetin in water and Fe³⁺ ions in several solvent systems was realized at a long maximum excitation wavelength of 358 nm.